What is PSSM and how do you feed the PSSM horse?

What is PSSM and how do you feed the PSSM horse?

What is PSSM?

PSSM is commonly categorized as a “metabolic disease,” but it is actually a genetic muscle disease. We are going to talk about it in this section, however, because the PSSM horse also has issues with insulin. The genetic defect actually causes an excess amount of glycogen to be formed in the muscle and that excess glycogen is going to interfere at normal muscle function eventually.

The cause of PSSM is a genetic abnormality of the enzyme that converts glucose to glycogen, where there is more glycogen produced than in normal which eventually causes problems with normal muscle function. An acute episode of PSSM is characterised by muscle soreness, stiffness in gait, muscle tremors and spasms.

So why do we call it a metabolic disease? Because these horses are also extremely sensitive to insulin. In contrast to the EMS horse, the PSSM horses is extremely sensitive to insulin. horse owners are going to have to be very careful with feeding high starch and sugar diets. That’s why it is sometimes said that PSSM is also a metabolic disease.

This disease is most commonly seen in quarter horses, paints, appaloosas, Belgian draft horses, warmblood horses and a couple other breeds.

However, in both animals we need to be cautious with feeding diets high in non structural carbohydrates (NSC).

Feeding the PSSM horse

Opposite to the ID horse, which is insulin resistant, the PSSM horse is extremely sensitive to insulin. Insulin encourages glucose uptake into tissues, stimulating glycogen formation. To avoid excessive glycogen formation, the first goal of the dietary program in the PSSM horse is to not provide too much glucose to be turned into glycogen. Secondly, avoiding the insulin spikes will stimulate this process. Hence, we want to feed these horses a diet low in starch and sugars. Similar to the ID horse, make sure to look both at the starch and sugar of your horse’s concentrates as well as the hay that you feed.


  • You want to try to pick a hay that is below 12% NSC, which you can estimate from a hay analysis
  • NSC is typically higher in younger grass and grass grown in spring/fall. NSC content is also highest after a long sunny day.
  • So be mindful about turning your horse out on pasture, if you do so, make sure to turn it out in the mornings.
  • Choosing a more mature hay will also be safer as the NSC content will drop when the plant matures
  • In contrast to what many people believe, alfalfa is actually a good hay for the PSSM horse because its NSC content is typically much lower than most grass hays. It does have an overall higher energy content, but this has little to do with the NSC content itself and more with the fact that it is more digestible than most grass hays. However, because of the higher energy content you typically need to feed less alfalfa compared to grass hays. A good way to still feed enough forage is to feed an alfafa/grass mix.


  • Suitable concentrates for the PSSM horse are obviously those that are low in NSC. It is important to note though, that it is not necessarily the % starch or % sugar on the bag that matters but how much you feed. Good options are feeds high in digestible fiber and maybe fat if more calories are needed, for example those horses that still compete. Beet pulp or stabilized rice bran are always good options.
  • Exercise + dietary management

TIP: Work with a nutritionist if needed to ensure you feed a safe amount while still meeting all nutrient requirements.

Even more so than for the EMS horse, the only way to properly manage the PSSM horse is through BOTH diet and exercise. It is through exercise that the horse will get rid of the excess glycogen in their muscles, which will help prevent stiffness and muscle soreness. The more the horse can move, the healthier it will be so provide as much free turn out and exercise as possible. Without the exercise component, any dietary management will have little to no effect. 

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